The present invention is directed to an improved method of producing adherent metal coatings on the surface of magnesium/zinc alloy substrates.
Plating on magnesium alloys has been used for a number of years. However, in order to obtain good adhesion of the metallic coating to the magnesium/zinc alloy substrate, numerous processing steps have generally been required.
An example of one process is described in U.S. Pat. No. 4,349,390 to Olsen et al., the subject matter of which is herein incorporated by reference in its entirety. The steps in this process are as follows:                1) Surface preparation by mechanical pretreatment;        2) Degreasing using organic solvents or alkaline cleaning solutions;        3) Activating the surface of the magnesium alloy substrate;        4) Chemical zinc precipitation by immersion plating in an alkali metal pyrophosphate solution containing zinc ions, preferably at temperatures above 60° C.; and        5) Electrolytic metal coating.        
Magnesium is a very active metal, and the pickling steps in the above described pretreatment sequences tend to open up any underlying porosity in the magnesium substrate. Thus, although an adherent deposit of copper may be subsequently obtained, the cosmetic appearance and corrosion resistance of coatings applied on top of this copper deposit tend to be very poor.
Traditionally, the only way to obtain plated magnesium articles of good cosmetic appearance and corrosion resistance is to apply a thick layer of copper and mechanically polish the article at this stage to seal any porosity. Subsequently, the coated articles must be re-racked and re-activated before plating with subsequent metals, such as nickel and chromium. This makes the production of plated magnesium articles very expensive, especially as any “polish through” of the copper during the polishing operation will render the article useless.
More recently, magnesium alloys which contain a significant proportion of zinc have been developed. These alloys are claimed to have superior casting qualities and reduced levels of porosity.
The inventors of the present invention have surprisingly found that these alloys can be processed for plating using an etch-free pre-treatment process, which eliminates the need for a pickling or activation stage in the plating process. Thus, the porosity of polished magnesium castings is not opened up and articles of excellent cosmetic appearance and good corrosion resistance can be obtained without any intermediate polishing operations on the copper deposit prior to nickel (or other metal) plating. This has obvious commercial advantages in terms of reducing the number of processing stages necessary to produce a high quality finished article.
Upon further investigation, the inventors of the present invention have also discovered that the presence of zinc in the cast article is not the only factor relevant to the level of adhesion obtained during the etch-free process sequence. Another critical factor for successfully processing the magnesium alloy article is the aluminum content of the magnesium alloy. High zinc alloys tend to have a low aluminum content. Aluminum is added to magnesium alloys to harden the casting and produce grain refinement, but also gives a long freezing range, which may increase casting porosity.
The inventors of the present inventions have found that in order to be able to process castings using the desired “etch-free” process of the invention, the aluminum content of the casting must be controlled. For example, in alloys containing 4% or more of zinc, it is desirable that the aluminum content be less than about 9% and in alloys containing less than 4% of zinc, it is desirable that the aluminum content be less than 6%.
Without wishing to be bound by theory, the inventors believe that this is due to the presence of intermetallic magnesium/aluminum phases precipitated at the surface during cooling from the melt in the casting process. These intermetallic phases then produce micro-galvanic effects during the pre-treatment and plating process which leads to poor adhesion unless pickling and activation stages are employed in order to equalize surface potential.
The inventors have determined that alloys having less than 6% zinc can be processed by applying the zinc coating in an immersion process, with a zinc processing solution containing pyrophosphate, fluoride and zinc. The inventors have also determined that when the alloy contains more than 6% zinc, superior results can be obtained in an electrolytic process, where the application of a cathodic current forces the zinc to deposit from the solution.